If moisture potential of soil can be accurately monitored, irrigation can be controlled to optimize the rate of plant growth. One type of instrument for measuring soil moisture potential is a tensiometer. A conventional tensiometer comprises a sealed tube defining a chamber which is normally completely filled with water, a hollow porous tip on one end of the tube, and a vacuum gauge connected to the water chamber. The porous tip is inserted in the soil, and establishes liquid contact between the water in the tube and moisture in the soil surrounding the tip. Relatively dry soil tends to pull water from the tube through the porous tip. However since the tube is sealed, only a minute amount of water is actually withdrawn. Accordingly, the water in the tube is placed under tension by the pulling effect of the dry soil, thus creating a measurable subatmospheric pressure in the tube. Higher moisture contents in the soil produce correspondingly less vacuum in the tube, and completely saturated soil registers substantially zero vacuum or atmospheric pressure.
Typical tensiometer constructions provide a tube or column of water which extends from the porous tip to above grade. It will be apparent that the deeper the porous tip is buried, the longer the column of liquid above it will become.
Air presence in the water reservoir during tensiometric measurement is undesirable. Air can enter the reservoir by diffusing through the porous tip. More commonly, dissolved air present in the water that enters the vessel comes out of solution in the reduced pressure environment of the tensiometer. Eventually, the entire tensiometer would become filled with air. This air will increase the time required to reach pressure equilibrium because large volumes of water must move through the porous tip to effect the mass transfer of air through the tip. Thus in order to obtain accurate readings, the water and air are desirably purged periodically from the tensiometer reservoir and replaced with degassed water.
To facilitate purging of air from the tensiometer reservoir, a conventional tensiometer is typically provided with a column of water connecting a surface located pressure measuring device to the soil-embedded porous tip. However, there is a physical limit to the length of a column of water which can be supported by atmospheric pressure (about 1000 cm at sea level), and the useful measurement range of the tensiometer is reduced as the column of water above the porous tip is lengthened. The pressure exerted by the column of water increases the pressure in the porous tip, which in turn increases the apparent soil moisture tension recorded by the above-surface pressure measuring device.